Scientists have discovered that caffeine loses its powers to give us that necessary jolt if we sleep for less than five hours a night on three consecutive nights. In other words, don’t bother drinking coffee after the third night of poor sleeping: it won’t make any difference.
Coffee “significantly improves” our performance levels if we haven’t slept well for two nights in a row, say scientists at the Walter Reed Army Institute of Research. But the benefits of consuming two 200 mg shots of caffeine a day don’t have any effect come the third day.
They discovered caffeine’s diminishing benefits when they tested it on a group of 48 volunteers, whose sleep was restricted to just five hours each night for five consecutive nights. They were then given the two shots of caffeine or a placebo (sugar pill), and their performance and alertness was measured afterwards.